1. Field of the Invention
The present invention relates to an image processing apparatus and an image processing method, and a printing apparatus and a printing system using the image processing apparatus. More particularly, the invention is suitably applied to a printing apparatus for printing a color image using a plurality of types of printing materials having different color tones, and an image processing method for the printing apparatus. The present invention may be applied not only to an ordinary printing apparatus, but also to any other appropriate apparatus, such as a copier, a facsimile apparatus having a communication system, a word processor having a printing unit, or the like, or to an industrial recording apparatus combined with various types of processing apparatuses.
2. Description of the Related Art
A printer is known as a printing apparatus for recording information, including desired characters, images or the like, on a sheet-shaped printing medium, such as paper, a film or the like, that serves as an information output apparatus for a word processor, a personal computer, a facsimile apparatus or the like.
Various types of conventional printing methods by a printer are known. Recently, ink-jet printing has become important because it has various advantages over other printing methods, such as, for example, the capability to perform non-contact printing on a printing medium, such as paper or the like, to easily form a color image, and to perform silent printing. A so-called serial method, in which printing is performed by mounting an ink-discharging print head in accordance with desired print information and scanning a printing medium, such as paper or the like, with the print head in a direction (a main scanning direction) orthogonal to a conveying direction (a sub-scanning direction) of the print medium, has been widely utilized as the configuration of a printing scanning system of the printing apparatus to which the ink-jet method is applied, because an inexpensive and small apparatus can be easily obtained.
In conventional ink-jet printing apparatuses, dots are recorded according to binary-encoding processing or multivalue-encoding processing.
Recently, as a result of increasing popularity and availability of digital cameras having pixels exceeding three million in number, and realization of high picture quality in image input apparatuses, such as high-resolution scanners and the like, the amount of data to be processed by an output apparatus is increasing, and high accuracy in printing is required for a high-resolution image.
FIG. 26 is a graph illustrating the human visual characteristic (a VTF curve) with respect to gradations. The human visual characteristic indicates the number of gradations that can be discriminated by a human being at a certain observation distance (25 cm). If the capability to reproduce gradations of a printing apparatus is superior to the human visual characteristic, it can be said that the printing apparatus has an excellent gradation reproducing capability.
As is apparent from FIG. 26, when comparing the gradation reproducing capability of a printer performing encoding processing to various values of the human visual characteristic, the range exceeding the human visual characteristic easily widens as the number of encoding values increases. For example, in order to provide a gradation reproducing capability equivalent to multivalue encoding (septenary encoding) for input data having a resolution of 600 ppi (pixels per inch, reference value), binary encoding is necessary for input data with a resolution of 1,500 ppi. Accordingly, the gradation reproducing capability can be easily widened by performing multivalue(septenary)-encoding processing for input data having a resolution of 600 ppi than by performing binary encoding processing.
Multivalue-encoding processing will be briefly described with reference to FIGS. 27A and 27B. If it is assumed that a printing apparatus has an output resolution of 1,200×1,200 dpi (dots per inch, reference value) in the main scanning direction and in the sub-scanning direction, in binary-encoding processing shown in FIG. 27A, an input resolution of 1,200 ppi is required, and two gradations are represented by presence/absence of formation of one dot for an input pixel having a size of about 21.17 μm ( 1/1,200 inch) square. On the other hand, in quinary-encoding processing shown in FIG. 27B, five gradations are represented by presence/absence of formation of 2×2 =4 dots for an input pixel having a size of about 42.33 μm ( 1/600 inch) square. Accordingly, when performing quinary-encoding processing, input data can be reduced to ¼ of that when performing binary-encoding processing. That is, for a request for a print having high picture quality, while a high input resolution is necessary and the amount of data increases in binary-encoding processing, a relatively low input resolution is necessary and the amount of data decreases as the number of encoding values increases.
Unevenness sometimes results, for example, when moving a print head in the main scanning direction, due to errors in production of a scanning system of a printer main body, and the like (hereinafter termed “main-body noise”). Furthermore, when providing two discharging-port columns for discharging ink in the main scanning direction by being shifted by ½ of the pitch of arrangement of discharging ports in the sub-scanning direction (the sheet feeding direction), causing discharging ports of respective columns to perform recording of even-numbered rasters and odd-numbered rasters, deviation (in even/odd registration) in the position of ink provision (the dot forming position) is sometimes produced between even-numbered rasters and odd-numbered rasters.
A printing apparatus performing binary processing tends to be influenced by such main-body noise and head noise. In a printer performing multivalue processing, however, as disclosed, for example, in Japanese Patent Application Laid-Open (Kokai) No. 2001-63016 (2001), by indexing a plurality of patterns, each for determining a dot arrangement at output within a M×N-dot region for one input pixel, and selecting an appropriate pattern, the influence of main-body noise and head noise can be reduced.
Because of the above-described reasons, in recent ink-jet printing apparatuses, multivalue processing using a dot-pattern index (hereinafter termed “index processing”) tends to be used instead of binary processing.
As described above, multivalue processing has advantages such as, for example, only a relatively low input resolution is required and the amount of data can be reduced, and the influence of main-body noise and head noise can also be reduced, compared with binary processing.
However, it is necessary to provide dot patterns, whose number equals the number of recording gradations, each of which determines dot arrangement in accordance with the number of recording gradations in a predetermined region. Furthermore, in order to mitigate the influence of main-body noise and head noise, it is necessary to provide a plurality of dot arrangement patterns in a predetermined region even at the same gradation. Still further, if the number of recording gradations is increased in order to improve the image quality, it is also necessary to increase the capacity of a memory for storing a table of patterns.
Recently, the use of a printer which provides not only a black or another monochrome print, but also a color print or a high-quality print using photo-ink materials (comprising four primary color ink materials and light-color ink materials of predetermined colors) has been generalized. In accordance with an increase in the types of ink materials, it is necessary to provide a dot-pattern table for each color tone, and a large-capacity memory (a ROM (read-only memory) or the like) for storing such tables, resulting in an increase in the cost of a printing apparatus. If the size of a dot-pattern table is simply reduced, banding unevenness between main scanning lines of the print head tends to be pronounced, thereby causing the possibility of a decrease in the image quality.